Cluster beam deposition is used to obtain model catalysts (palladium on highly oriented pyrolitic graphite). The supported particles are characterized by transmission electron microscopy (TEM) and scanning tunneling microscopy. Decoration of graphite steps and formation of groups of palladium particles on the graphite surface are imaged by TEM. High spatial resolution observations show periodic (square-root 3 X square-root 3)R30-degrees charge density modulations of graphite around the supported particles. Such modulations are explained following the work of Mizes and Foster [Science 244, 559 (1989)] that take into account the fact that the particles act as perturbations to the graphite wave functions. On the basis of this explanation, the interactions of palladium and platinum (Xhie et al., Phys. Rev. B 43, 8917 (1991)] with the HOPG are compared. The results of this comparison are in complete agreement with the different catalytic behavior of the two metals in presence of reactions of hydrogenation of unsaturated hydrocarbons.